1. Field of the Invention
The invention relates to a tissue compensation system and method for use its with a linear accelerator to provide radiation therapy to a portion of a patient, whereby some of the radiation beam impinging on thinner areas of tissue within a treatment area are absorbed by the tissue compensator.
2. Description of the Prior Art
Many hospitals provide radiation therapy to their patients, particularly older patients for whom cancer has become an ever growing problem. An area which is of growing interest in the field of radiation therapy is that of tissue compensation, or the practice of absorbing some of the radiation beam impinging on thinner areas of tissue within a treatment area. Tissue compensation permits a higher effective dose of radiation to be delivered to thicker tissue areas without overdosing the areas of "missing" tissue, or the thinner areas of tissue. The difference in dosage that can be delivered can amount to as much as thirty percent depending upon the differences in thickness and the power of the linear accelerator employed to provide the radiation therapy.
Several systems are known which assemble varying thicknesses of metal compensatory blocks which are interposed in the path of the radiation treatment beam. This process usually involves two steps, wherein information is first obtained about the contour of the treatment surface on patient's body which is to be exposed to the radiation beam, which information indicates how much tissue is "missing" at any given spot on the contoured surface. In the second step, the information obtained is used to generate a tissue compensator, assembled from small blocks, employed at the time of patient treatment. In one system, patient contour information is obtained by reading the position of a tracing stylist directed on the patient's body as reflected in a magnetic field generated by a mechanism under the table upon which the patient rests. This contour information is fed to a three-axis CNC milling machine which is used to generate a compensatory pouring mold which is later filled with a metal alloy. At the end of the procedure the metal alloy is recovered and the styrofoam mold which has been prepared is discarded.
In another system, a light-dot pattern is projected on the treatment area on the contoured surface of the patient. The distortion of each light-dot is quantified by a computer, which then generates the information necessary to construct a compensator out of aluminum and brass blocks.
The foregoing systems have the disadvantages of: being very expensive; requiring sophisticated computer interfaces to obtain the necessary information relating to the contour of the treatment area on the patient's body; requiring valuable technician time to create the compensator; and requiring floor space, which is in sometimes short supply in a hospital or a radiation therapy center, to store the various components of the systems.
Another approach to obtaining information about the contour of a surface to be treated on the patient has been the use of a box containing a plurality of rods which can project outside the box. The box may be laid against the patient's body to indicate differences in tissue depth as a function of the location of the rod with respect to the box. The differences in tissue depth, or the differences in lengths of the rods once they are laid against the body, are then measured and that information is used to build a compensator. The disadvantages associated with the use of such a box include the following problems.
The basic problem is one of spatial resolution. The treatment area on a surface of the patient's body for which tissue compensation might be needed could extend over a square surface having outer dimensions of from 20 to 30 centimeters on some patients. In order to be of use to properly obtain information from which to prepare a tissue compensator, such boxes must have the capability of recording differences of tissue depth for every square centimeter on the treatment surface. For the maximum size treatment area previously described, it would then be necessary to use 400-900 rods for which the linear displacement of each rod extending out of the box would need to be measured and recorded. The time and physical manipulation necessary to measure and record the lengths of so many rods is prohibitive, and it is questionable as to whether or not it is even possible. Such prior art boxes did not include enough rods to be capable to record differences for every square centimeter on the treatment surface. In addition, the likelihood of introducing errors when one is to measure, record, and transfer such a quantity of information manually makes such an approach subject to errors, which could create substantial risks associated with the radiation therapy using compensator based upon such information.
Another disadvantage associated with such a prior art box is that the information obtained from measuring the displacement of the rods cannot be readily used as it is measured. The radiation beam exiting a point source at the head of the radiation therapy machine, such as a linear accelerator, is a divergent beam which spreads outwardly prior to striking the treatment surface on the patient. Accordingly, the compensation required at the head of the linear accelerator, where such compensators are mounted, is a compacted form of the actual measurements generated at the treatment surface of the patient. Such prior art boxes including a plurality of rods measured the rod displacement in a plane perpendicular to the treatment surface. Accordingly, it would be necessary to take into consideration the divergent nature of the radiation beam, as it affects measurements obtained from such a prior art box, and such prior art boxes did not take this factor into account.
Accordingly, prior to the development of the present invention, there have been no tissue compensation methods and apparatus which: are simple to understand and easy and economical to use; sensitive enough to permit depth information to be obtained for every square centimeter on the patient's treatment surface; take into account the diverging nature of the radiation beam; require as little transfer of data as possible, so as to minimize the chance of introduction of measurement errors; do not require any ancillary equipment such as computers, milling machines, etc., all of which add additional costs in terms of money, space, and personnel requirements; and respect the environment by not utilizing disposable components and permit all parts to be reused.
Therefore, the art has sought a tissue compensation method and apparatus which: are simple to understand and easy and economical to use; are sensitive enough to obtain tissue depth information for every centimeter on the treatment surface; account for the diverging nature of the radiation treatment beam; require as little data as possible, so as to minimize the chance of the introduction of errors; do not require any ancillary equipment such as computers, milling machines, etc., so as to not add any additional cost in terms of money, space, and personnel requirements; and are respectful of the environment by not requiring the use of disposable components and permit all of its components to be reused.